Materials which closely mimic the ultrasonic propagation characteristics of human tissue are employed in imaging phantoms and other test objects for use with ultrasound scanners. These phantoms may be used to carry out performance checks on ultrasound scanners. Phantoms may also be used for training or testing student technologists in the operation of ultrasound scanners or the interpretation of ultrasound images produced by such scanners.
Ideally, such material should be capable of mimicking soft human tissue with respect to at least three characteristics: speed of sound, ultrasonic attenuation, and ultrasonic scattering. The speed of sound in the tissue mimicking material should rest in the range from approximately 1460 m/s, characteristic of human fat tissue, to 1640 m/s, characteristic of the human eye's lens. The attenuation coefficient with respect to frequency of the material should lie in the range from approximately 0.4 dB/cm/MHz, characteristic of human fat tissue, to 2.0 dB/cm/MHz, characteristic of human muscle tissue. Additionally, the attenuation coefficient should be approximately proportional to the ultrasonic frequency. In other words, the attenuation coefficient with respect to frequency, or the attenuation coefficient slope, should remain constant for varying ultrasonic frequencies. These characteristics of human tissue should be maintained at all frequencies in the typical range of ultrasonic scanners, from 1-10 MHz. Moreover, the variation of these characteristics within the range of room temperature should be small. Additionally, these materials should be stable in time and invulnerable to reasonable environmental fluctuations. They should also be free of any pockets of air or gas. Furthermore, the bulk properties of the material should be the same throughout the volume of a particular phantom or phantom section.
A tissue mimicking material satisfying the above characteristics was disclosed in U.S. Pat. No. 4,277,367, to Madsen, et al., entitled Phantom Material and Method, the disclosure of which is incorporated herein by reference. In that patent it was disclosed that both the speed of sound and the ultrasonic attenuation properties, matching those of most soft tissues, could be simultaneously controlled in a mimicking material based on water based gels, such as those derived from animal hides. It was disclosed that pharmaceutical gels could be made to mimic human soft tissue from the standpoint of attenuation coefficient by having a uniform distribution of graphite powder in the gel, and that the magnitude of the attenuation coefficient could be controlled by varying the concentration of graphite. The speed of sound in such a gel suspension could be varied between 1560-1700 m/s at room temperature and was independent of the graphite concentration. The attenuation coefficient was also nearly proportional to the frequency. In one embodiment of that invention, ultrasound phantoms embodying the desired features for mimicking soft tissue were prepared from a mixture of gelatin, water, n-propanol and graphite powder, with a preservative. In another embodiment, an oil and gelatin mixture formed the basis of the tissue mimicking material.
Tissue mimicking material is typically used to form the body of an ultrasound scanner phantom. This is accomplished by enclosing the material in a container which is closed by an ultrasound transmitting window cover. The tissue mimicking material is admitted to the container in such a way as to exclude air bubbles from forming in the container. In addition to the tissue mimicking material itself, scattering particles, spaced sufficiently close to each other that an ultrasound scanner is incapable of resolving individual scattering particles, and testing spheres or other targets may be located within the phantom container, suspended in the tissue mimicking material body. Such an ultrasound phantom is useful in evaluating the ability of ultrasound medical diagnostic scanners to resolve target objects of selected sizes located throughout the tissue mimicking material. The objective is for the ultrasound scanner to resolve the testing spheres or other targets from the background material and scattering particles. This type of ultrasound phantom is described in U.S. Pat. No. 4,843,866, to Madsen, et al., entitled Ultrasound Phantom, the disclosure of which is incorporated herein by reference.